When displaying an image picked up by a digital camera on the display screen of a monitoring apparatus, it is necessary to convert the data format of the image data of the digital camera into a format adapted to display the image on the screen of the monitoring apparatus. Normally, the data format of the image data of the digital camera is converted by inserting a converter board having a data format converting feature between the digital camera and the monitoring apparatus. Such a converter board can be built in a personal computer, so as to perform a processing operation on the image data transmitted from the digital camera for format conversion and transfer the image data to the monitoring apparatus. With this arrangement, it is possible to provide a system that allows smooth image pickup operations.
FIG. 1 of the accompanying drawings schematically illustrates the circuit configuration of a converter board 110 having such a data format convening feature. Referring to FIG. 1, the converter board 110 is connected to a digital camera adapted to output image data in a format conforming to the IEEE 1394 Standard and comprises a CPU 111, a CPU bus 112, a system ROM 113, a system RAM 114, a PCI bus 115, a PCI bridge circuit 116, a graphic control circuit 117 and a graphic memory 118. An IEEE 1394 control board 120 for controlling data transmission between the converter board 110 and the digital camera (not shown) is also connected to the PCI bus 115.
The CPU 111 controls the component sections of the converter board 110 by way of the CPU bus 112 according to the programs stored in the system ROM 113 and executes various processing operations. The system ROM 113 typically comprises a flash ROM and stores various programs and fixed data necessary for the operations of the CPU 111. The system RAM 114 typically comprises an SDRAM and temporarily stores data necessary for various control operations of the CPU 111. The PCI bus 115 is used for transmitting data between the CPU 111 and peripheral elements. The PCI bridge circuit 116 controls the connection between the PCI bus 115 and the CPU bus 112.
The graphic control circuit 117 writes image data in and reads image data from the graphic memory 118 having a storage area for storing data of a frame on a time division basis, the data originating from the digital camera. In other words, the graphic control circuit receives the data transmitted from the CPU 111 by way of the PCI bridge circuit 116 and the PCI bus 115 and writes the data in the graphic memory 118. The graphic control circuit 117 also reads the image data stored in the graphic memory 118 and transmits them to the monitoring apparatus by way of the connector 119 and a cable (not shown).
The IEEE 1394 control board 120 comprises an IEEE 1394 physical layer control circuit 121, a link control circuit 122 and a PCI bridge circuit 123.
The physical layer control circuit 121 is a so-called PHY chip for controlling IEEE 1394 physical layers and adapted to convert the analog signal input from an IEEE 1394 cable (not shown) by way of the connector 124 into digital data. The link control circuit 122 is a so-called LINK chip for controlling IEEE 1394 link layers and adapted to control the interface between the physical layer control circuit 121 and the PCI bridge circuit 123. The PCI bridge circuit 123 controls the connection of the link control circuit 122 and the PCI bus 115.
For transferring the image data input from the digital camera to the monitoring apparatus by means of the converter board 110, firstly the image data received by the IEEE 1394 control board 120 is taken into the CPU 111 by way of the PCI bridge circuit 123, the PCI bus 115, the PCI bridge circuit 116 and the CPU bus 112 and its format of the image data is converted into the format to be used for displaying an image on the display screen of the monitoring apparatus by means of a processing operation using the software stored in the CPU 111.
The image data whose format is converted by the CPU 111 is then transmitted to the graphic control circuit 117 by way of the CPU bus 112, the PCI bridge circuit 116 and the PCI bus 115 and finally sent to the monitoring apparatus by means of the graphic control circuit 117 and the graphic memory 118.
Meanwhile, when converting the format of the image data by means of the software stored in the CPU 111, the processing operation of the CPU 111 is slow relative to the data transmission rate for IEEE 1394 isochronous data to give rise to a problem of a reduced frame rate for the image to be displayed. Additionally, there is a problem of a reduced processing rate of the entire system because of a lowered operational capacity of the system due to the increased processing load of the CPU 111 caused by the data transmission.
Furthermore, with the above described known converter board, the transmission rate of the image data input from the digital camera and that of the image data transmitted to the monitoring apparatus can be different. In most cases, this difference is caused by the difference between the rate of writing data in and that of reading data from the graphic memory. Therefore, when the image data of a frame is written into and read from the storage area of the graphic memory for a frame and there arises a difference between the rate of writing image data and that of reading image data, there can arise a phenomenon that the lines being read out can outrun the lines being written in. Such a phenomenon can result in line noise (outrun scanning noise) that appear on the image being displayed on the screen of the monitoring apparatus. Particularly, when a moving image is continuously transmitted and displayed on a real time basis, such an outrun scanning noise can periodically occur to degrade the image quality.
The above identified problems need to be dissolved for any systems adapted to transmit image data input from a digital camera to a monitoring apparatus and those adapted to transmit image data input from an image processing device to an electronic device.